1. Field of the Invention
The present invention relates generally to an optical storage medium such as an optical disc and to a manufacturing method for the same. More specifically, this invention relates to the structure of a high density, multiple layer optical storage medium having plural data recording layers with a guide groove, and to a manufacturing method for this optical storage medium.
2. Description of Related Art
Compact Discs (CDs) are now widely used and optical discs have earned an important role as a recording and storage medium. While conventional CD media is read-only, CD-R (recordable) and CD-RW (rewritable) media can also be used by the end user for recording and reproducing information. Subsequent research continues to develop other types of even higher density optical discs.
Optical discs have a guide groove consisting of a land and groove pattern for signal recording formed in the data recording layer. When seen from the side emitting the laser beam for recording or reproducing data, the side that is recessed in a concave shape away from the laser beam source is called the land, and the side that protrudes toward the laser beam source is called the groove. Recording capacity is determined in part by the pitch of the recordable and reproducible guide grooves (the track pitch). DVD-RAM media, for example, uses a land and groove recording method to record data to both lands and grooves. The track pitch is 0.74 xcexcm in DVD-RAM media with a 2.6 GB recording capacity, and is 0.615 xcexcm with a 4.7 GB recording capacity.
Optical disc recording density can be increased by increasing recording density in the recording layer and by increasing the number of recording layers. For example, Digital Versatile Disc (DVD) media, which has been standardized and are now readily available, is a read-only optical disc media with two data recording layers formed so that both data recording layers can be read from the same side of the disc. An optical disc having two data recording layers that can be recorded by an end user has also been developed and reported in the literature for use as recordable and readable DVD media.
The structure of a recordable and readable optical disc is described next with reference to FIG. 2. The recordable and readable optical disc shown in FIG. 2 has two data recording layers. Both data recording layers are made using a phase change recording material, a type of material whose optical characteristics can be changed between amorphous and crystalline states. Data is recorded to the data recording layers by recording a pattern of marks. More specifically, a first data recording layer 206 is formed in a first transparent layer 201 having a guide groove. The first data recording layer 206 consists of a recording film 203 made from a phase change material disposed between substantially transparent dielectric film 202 and substantially transparent dielectric film 204, and a translucent metal reflection film 205.
The second data recording layer 212 is formed in a second transparent layer 207 having a guide groove. This second data recording layer 212 consists of a metal reflection film 211 and a recording film 209 disposed between substantially transparent dielectric film 210 and substantially transparent dielectric film 208.
The first data recording layer 206 and second data recording layer 212 are bonded together separated a specific distance by an intervening substantially transparent adhesive layer 213. These data recording layers are formed following the guide groove patterns on the substrate surfaces, the recording film of each data recording layer is also formed following the guide groove pattern. The recording films therefore also have a guide groove.
The recording marks are formed in the recording film of the data recording layer, and the recording and reproducing characteristics are therefore more easily affected by the guide groove pattern on the recording film surface than the guide groove pattern on the substrate surface. In a conventional two-layer disc the guide groove width is wide compared to the thickness of the data recording layer, and two-layer optical discs are therefore produced according to the shape of the guide grooves formed on the substrate surface.
In order to further increase the recording density of the data recording layers, the guide groove pitch must be reduced and the guide grooves must be made accordingly narrower. However, if the ratio of the guide groove width to the data recording layer thickness (particularly the thickness of the film between the substrate surface and the recording film) is small, the width of the guide grooves appearing on the recording film surface when the recording film is formed will be small relative to the width of the guide grooves on the substrate surface. A difference also develops between the film thickness in the substrate guide grooves and the film formed between the guide grooves, which causes guide groove depth to change in addition to the guide groove width.
Because the configuration or thickness of each data recording layer differs in a multilayer optical disc, the desired performance cannot be achieved if the guide groove width and depth are the same in each layer even if the recording density is the same in each data recording layer.
An object of the present invention is therefore to provide a high storage capacity, high density optical disc by changing the width and/or depth of the guide grooves on the substrate surface in each data recording layer, and forming the depth of the guide grooves on the recording film surface to a desired shape in each data recording layer.
To achieve this object, an optical disc according to the present invention has plural substrate layers each having plural guide grooves; plural data recording layers laminated on the substrate layers, each of the plural data recording layers having a recording film for recording data over the guide grooves; and an intermediate layer between the plural data recording layers. The guide groove width is different on each substrate layer. The guide grooves of the data recording layers formed according to the guide grooves of the substrate layer have the same pitch in each data recording layer.
Further preferably, light for recording and reproducing data is emitted to one side of the optical disc, and the groove width of the guide grooves widens with the depth to the substrate layer from the surface to which the light is emitted.
An optical disc manufacturing method according to the present invention for manufacturing an optical disc having a plurality of substrate layers each having a recording film for recording data has steps for: supplying a plurality of substrates; coating each of the plural substrates with a photosensitive material; recording a pattern containing guide grooves in each of the plural substrates by changing guide groove recording beam density so that the recorded guide groove width is different in each of the plural substrates; developing each of the plural substrates to produce a plurality of master plates having the recorded pattern; duplicating a plurality of stampers based on each of the plural master plates, and producing a plurality of substrate layers based on the stampers; laminating a data recording layer having a recording film on each of the plural substrate layers; and bonding the plural data recording layers together by way of an intervening intermediate layer.
Preferably, an optical disc according to the present invention is recordable and reproducible using 390 nm to 450 nm wavelength light emitted from a read/write head having an objective lens with a numerical aperture of 0.7 or greater. This optical disc has a substrate layer having guide grooves at a track pitch of 0.36 xcexcm or less; a data recording layer containing a recording film laminated to the substrate layer for recording data to a guide groove of the recording film formed according to a guide groove of the substrate layer; and a transparent layer that is substantially transparent laminated to a thickness of 0.3 mm or less to the data recording layer for passing light emitted to the data recording layer. The groove width of the guide groove formed on the substrate layer is wider than the groove width of the guide groove of the recording film and within 50% to 80% of the track pitch.
It is possible with a multilayer optical disc having plural data recording layers with guide grooves according to the present invention to optimize the guide grooves in the data recording layers to substantially the same the width or depth by changing the width or depth of the guide grooves at the substrate surface for each data recording layer. This is achieved by changing the guide groove width or depth in the master that is the basis for the guide groove pattern on the substrate surface. Good playback signal quality can therefore be achieved from each data recording layer when a signal is recorded and then reproduced.
The groove width of the guide grooves formed in the substrate layer is greater than the groove width of guide grooves in the recording film and is from 50% to 80% of the track pitch in an optical disc according to the present invention. This achieves a high density optical disc with a large storage capacity and excellent recording and reproducing characteristics enabling information to be recorded and reproduced. This optical disc is achieved by adjusting the groove edge angle of the guide grooves to a desirable angle.
Furthermore, by using the narrowing of the groove width resulting with film formation as a result of building up a thin film layer including a recording film layer in the trench part of the land and groove pattern in both single-sided, single layer optical discs and single-sided two-layer optical discs, the present invention can also achieve a single-sided two-layer optical disc using currently developed laser beam recorders and molding methods.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.